In a conventional electro-optical distance meter, errors unique to the electro-optical distance meter have been corrected by measuring the distance by alternately switching, by moving a shutter, light emitted from a light-emitting element into a distance measuring optical path for traveling to and from a target reflection object (a target, reflective sheet, or non-prism object) and into a reference optical path directly heading for a light receiving element from a light source. However, because a shutter movement is involved, there have been drawbacks that not only does the distance measurement take time, but also the motion is slowed down at low temperature. Therefore, an electro-optical distance meter without using a shutter for switching into a distance measuring optical path and a reference optical path has been desired.
As such an electro-optical distance meter without using a shutter, one as disclosed in the following patent literature 1 is known. In FIG. 9, a block diagram of this electro-optical distance meter is shown.
This electro-optical distance meter includes two light-emitting elements 1P and 2P. A light 30P emitted from the light-emitting element 1P is split into two parts by a beam splitter 3P, one of these is as a distance measuring light 32P made incident onto a light receiving element 5P through a distance measuring optical path for traveling to and from a target reflection object 60P, and the other is as a reference light 33P made incident onto a light receiving element 4P through a reference optical path that is inside the electro-optical distance meter. A light 31P emitted from the light-emitting element 2P serves as a reference light, and is split into two parts by a beam splitter 6P, one of these is made incident onto the light receiving element 4P, and the other 35P is made incident onto the light receiving element 5P. In addition, a diffuser 51P for diffusing light is disposed at a nearer side of the light receiving element 4P, and a scatter 11P for scattering light is disposed at a nearer side of the light receiving element 5P.
The light-emitting element 1P is connected to a synthesizer 21P via an amplifier 23P, and emits light modulated with a frequency f1. The light-emitting element 2P is connected to a synthesizer 22P via an amplifier 24P, and emits light modulated with a frequency f2. Both synthesizers 21P and 22P are connected to a common oscillator 20P.
The light receiving element 4P is connected to a frequency converter 7P via an amplifier 9P, and the light receiving element 5P is connected to a frequency converter 8P via an amplifier 10P. Both frequency converters 7P and 8P are supplied with a local oscillation signal of a frequency fLO from a local oscillator 12P. Both frequency converters 7P and 8P are for conversion to intermediate frequency signals fZF1 and fZF2 of frequencies equal to differences between output signals from the light receiving elements 4P and 5P and the local oscillation signal, respectively.
A filter 13P connected to the frequency converter 7P is structured so as to pass only a frequency fzf1 that is a difference between the frequencies f1 and fLO, and selects only an intermediate frequency signal according to the reference light 33P modulated with the frequency f1, that is, an intermediate frequency signal according to a reference distance D1. A filter 14P connected to the frequency converter 8P is structured so as to pass only a frequency fzf2 that is a difference between the frequencies f2 and fLO, and selects only an intermediate frequency signal according to the reference light 35P modulated with the frequency f2, that is, an intermediate frequency signal according to a reference distance D2+D3.
Next, the light-emitting elements 1P and 2P are changed in their respective modulation frequencies of emitting lights so as to emit light modulated with the frequency f2 from the light-emitting element 1P and emit light modulated with the frequency f1 from the light-emitting element 2P. Then, the filter 13P connected to the frequency modulator 7P, which is structured so as to pass only the frequency fzf1 that is a difference between the frequencies f1 and fLO, thus selects only an intermediate frequency signal according to the reference light 34P modulated with the frequency f1, that is, an intermediate frequency signal according to a reference distance D2. The filter 14P connected to the frequency converter 8P is structured so as to pass only the frequency fzf2 that is a difference between the frequencies f2 and fLO, and selects only an intermediate frequency signal according to the distance measuring light 32P modulated with the frequency f2, that is, an intermediate frequency signal according to a measuring distance D0.
Thus, by alternatively changing the modulation frequencies f1 and f2 of light to be emitted from both light-emitting elements 1P and 2P, a total of four intermediate frequency signals of the intermediate frequency signal according to the measuring distance D0 and the respective intermediate frequency signals according to the reference distances D1, D2, and D2+D3 are obtained.
The filter 13P is connected to an A/D converter 17P via an amplifier 15P. The filter 14P is connected to an A/D converter 18P via an amplifier 16P. Both A/D converter 17P and 18P are connected to a digital Fourier transformer 19P. With this, by determining initial phases of the four intermediate frequency signals, an error content unique to the electro-optical distance meter can be corrected to determine a precise distance to a measurement target 60P.